Automatic fidelity control circuits



April 11, 1939. R. A. BRADEN 2,153,738

' AUTOMATIC FIDELITY CONTROL ciacurrs Original Filed July 29, 1953 7 RECT/F/EQ I 1 in J76 INVENTOR RENE BRADEN ATTORNEY Patented Apr. 11, 1939 UNITED STATES PATENT OFFICE AUTOMATIC FIDELITY CONTROL CIRCUITS Rene A. Braden, Collingswood, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Original application July 29, 1933, Serial No. 682,743, now Patent No. 2,088,229, dated July 27, 1937. Divided and this application January 28, 1937, Serial No. 122,714

4 Claims.

itself, however,-as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into efiect.

Referring now to the accompanying drawing,

wherein there is shown in highly conventional,

form a radio receiving system embodying one form of my inyention, it may be stated that the receiving circuit shown in the drawing embodies means for varying the selectivity of a receiving set, and which means can be operated by a signalactuated control tube. More specifically, two coupled tuned circuits I and 2 are connected to a three-plate variable condenser 3 which adds capacity to one of the tuned circuits when removing capacity from the other. Hence, by rotating the rotor 4 of the condenser 3 the tuned circuits can be detuned in opposite directions.

By detuning two circuits equal amounts in opposite directions, the mid-frequency of the resonance curve is held constant while. the resonance curve is made broader, thus decreasing the selectivity and improving the fidelity. At the same time the signal output .is decreased, as the amplification is decreased when the circuits are detuned. This produces some volume control action, and 'may assist the regular automatic yolume control to extend "the signal intensity range over which it can control the output, or if applied to several intermediate frequency and/or 45 radio frequency stages, it may provide all the automatic volume control action needed, in addition to controlling the fidelity and selectivity.

The rotating plate, or rotor, 4 is mechanically coupled to the moving coil of a galvanometeri, 50 the latter carrying the direct current component of the plate current of acontrol tube, which tube may be the rectifier of the receiver, or signal demodulator, for example. Varying the amplitude of the carrier impressed on the rectifier varies the 55 Ip of the rectifier, swings the galvanometer coil,

the strength of this carrier.

and changes'the tuning of the coupled t'unedbircuits I and 2.

I of the demodulator (diode or triode) is more or less proportional to the strength of the carrier impressed on the demodulator, so that the posi- 5 tion of the movable condenser plate varies with The output of the rectifier, or demodulator, may be amplified by a direct current amplifier, and the output of the direct current amplifier can be used to operate the galvanometer and condenser. In that case, ofcourse, the bias of the control tube is varied.

It is believed sufficient to show and describe in purely conventional manner the circuit ele- I ments of the receiving system in the drawing which are not essential to a proper understanding of the present invention. For this reason the source; of signal energy is conventionally represented as feeding into-a stage of amplification 6. the coupled tunable circuits I and 2 being connected between the amplifier 6 and the succeed- The rectifier 8 may be the customary detector of the receiver, or it may be an auxiliary rectifier deriving its signal input energy from the network which feeds the receiver detector. The galvanometer may be operated by an automatic volume control tube, when a separate tube for this purpose is provided. Furthermore, it is to be clearly understood that stages 6 and I may be operating at intermediate frequency as in a superheterodyne receiver, or they may be cascaded amplifier stages of a tuned radio frequency amplifier receiver.

The reference numeral B designates the anode potential source for the anode of rectifier 8, the

- negative side of this source being connected to ground through a resistor R, the circuit between the anode of the rectifier and the positive side of the source B including a choke and condenser network to block the fiow of high frequency energy through the source B. The galvanometer 5 may be a milliammeter, one terminal of which may be grounded, and the other terminal of which adjustably connected, as by a lead 9, to the negative side of resistor R.

' The dotted line I I] represents a mechanical coupling, such as a shaft, between the grounded metallic rotor 4 and the moving coil II of the milliammeter 5, An indicator needle I 2 is shown affixed to the moving coil II, this needle cooperating with the index marks I3 to indicate the degree of selectivity adjustment. The tuned circuits I and 2 are resonated to the desired carrier frequency by the tuning condensers I4 and I5, 55

and the circuits I and 2 are magnetically coupled, as at M, the coupling being less than critical.v

One of the stator plates 4' of the control condenser 3 is connected to the high potential side of tuned circuit I, while the other stator 4" is connected to the high potential side of the other tuned circuit 2. The tuned circuits l and 2 are tuned to the desired carrier frequency when the rotor plate 4 is at its center position as shown in the-drawing.

Starting with the rotor plate 4 in its center position, as described, only a 90 rotation can be used. However, the galvanometer can operate through only 90, so this is good enough. The condenser can be made with quarter-circle plates instead of half-circle plates. That is, the stators 4' and 4" may be made of quarter plates, and the rotor 4 may be a quarter plate. This saves space and provides only asmuch condenser plate as is actually used. With no signal, the galvanometer is undefiected, and the rotoris in one extreme position. With increasing signal strength, the rotor turns a maximum of 90, which gives the full range of variation provided the signal strength at which action begins may be set at any desired level by adjusting the spring bias of the galvanometer. For example, the springs which oppose the action of the current inthe galvanometer coil may be tightened so that the galvanometer does not beglnto turn until, say, a current of 5 milliamperes is flowing. In this way the system can be made to operate only on signals which are well above the noise level, in which case an improvement in fidelity is desirable.

Since the rectifier plate current is proportion to the signal strength, an increase in the signal strength results in-the rotation of rotor plate 4, with the result that the circuits I and 2 are detuned in opposite directions to broaden the tuning and reduce the gain. Of course, this is what is desiredbecause when receiving strong signals, as from local stations, it is desired to have the fidelity of the receiver at its best, hence the selectivity of the receiver need not be at a maximum. When receiving weak signals, as from distant stations, the rectifier plate current aifecting the control meter 5 is at a minimum with the result that the rotor plate 4 is at its center position, thereby making the selectivity characteristic of the coupled tuned circuits sharp, the fidelity at that point of course being poorest.

If the current in the galvanometer exceeds the amount required to give maximum broadening of the circuits, thegalvanometer strikes a stop, and for all values of signal above this, the fidelity and selectivity remain constant. When tuning in a strong signal, the action of the condenser 3 makes the receiver appear to be very broad by means of its automatic volume control action, if the tuning is done slowly enough to allow the galvanometer to follow the signal strength variation. In this case the pointer on the galvanometer can be used as a tuning indicator. If the signal is much stronger than just enough to deflect the galvanometer to its extreme position, the variable tuning system is, in effect, locked in a fixed position, and there is no automatic volume control effect to make tuning diilicult. In this case the pointer does not operate as a tuningindicator, as it is held against the stop, and does not respond to signal variations.

While I have indicated and described a system for carrying my invention into eifect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described, but that many modifications. may be made without departing from the scope of my invention, as set forth in the appended claims. I

What I claim is:

1. In a signal transmission system of the type including a source of modulated carrier energy, a load circuit and a network coupling said source and load circuit, said system being characterized by the coupling network comprising at least two resonant circuits connected in cascade by predetermined fixed magnetic coupling, each circuit being tuned to the operating carrier frequency, means operatively associated with each of said resonant circuits for regulating the resonance curve characteristic of each resonant circuit, means, responsive to carrier amplitude variations, for automatically controlling the said first means in a sense such that the over-all resonance curve characteristic of the coupling network is broader for strong carrier amplitudes than for weak amplitudes without change in said operating frequency, said resonance regulating means consisting of a variable condenser including at least two stators and a rotor, one stator being connected to a point of high alternating potential of one of the resonant circuits, the other stator being connected to a point of high alternating potential of the other resonant circuit, the rotor being maintained at an alternating potential less than said two points, and said rotor being adjustable with respect to the stators by said automatic control means.

2. In a signal transmission system of the type including a source of modulated carrier energy, a load circuit and a network coupling said source and load circuit, said system being characterized by the coupling network comprising at least two resonant circuits connected in cascade by predetermined fixed magnetic coupling, each circuit being tuned to the operating carrier frequency, means operatively associated with each of said resonant circuits and independent of said magnetic coupling for regulating the resonance curve characteristic of each resonant circuit without varying the magnitude of said magnetic coupling, and means, responsive to carrier amplitude variations, for automatically controlling the said first means in a sense such that the over-all resonance said reactance means being constructed and arranged for adjustment in such a manner that it increases the frequency of one of the resonant circuits upon decreasing the frequency of the other resonant circuit and without aiifecting the said magnetic coupling, and means, responsive to wave'amplitude variation, for automatically controlling the adjustment of said common reactance.

4. In a high frequency wave transmission networkcomprising at least two resonant circuits coupled together by magnetic coupling of prede- I termined fixed value, and wherein said resonant increases the frequency of one 01 the resonant i circuits upon decreasing the frequency or the other resonant circuit and without aflecting the said magnetic coupling, means, responsive to wave amplitude variation, for automatically controlling the adjustment of said common reactance, and said common reactance comprising an 5 adjustable condenser whose rotor position is controlled by said wave amplitude responsive means.

- RENE A. BRADEN. 

